The present invention relates to a light-emitting element and a method of fabrication thereof using nitride semiconductors and, in particular, to a light-emitting element and a method of fabrication thereof that make it possible to extract light of an extremely small spot size.
It has recently become known to use nitride semiconductors such as GaN as materials for light-emitting diodes and semiconductor lasers in the wavelength region from blue to ultraviolet. These materials are attracting attention because they have direct-transition band structures and can achieve high light-emitting efficiencies. In particular, research and development is proceeding on semiconductor lasers using nitride semiconductors, which emit light of an extremely short wavelength on the order of 400 nm, so they can be expected to act as light sources for reading and writing data with respect to high-density optical discs having a capacity of at least 15 gigabytes per side.
Note that the term xe2x80x9cnitride semiconductorsxe2x80x9d as used in this document comprises semiconductors of all compositions given by the chemical formula InxAlyGazN (where xxe2x89xa61, yxe2x89xa61, zxe2x89xa61, and x+y+z=1), where each of x, y, and z is varied throughout its respective range. For example, InGaN (where x=0.4, y=0, and z=0.6) is comprised within the term xe2x80x9cnitride semiconductors.xe2x80x9d Furthermore, semiconductors wherein part of the indium, aluminum, or gallium, which are elements of the group III, is replaced with boron (B) or part of the nitrogen, which is an element of the group V, is replaced with arsenic (As) or phosphorous (P) are also comprised therein. In this case, semiconductors comprise any one of the three elements (In, Al, and Ga) listed above as group III elements and always comprise the nitrogen (N) as a group V element.
In such a semiconductor laser, the light emitted from the lasing region, in other words, the light-emitting portion thereof, expands rapidly with distance therefrom. When such a laser is used as a light source of an optical disc system, the light must be focused with a lens.
However, there are problems in that it is difficult to design the diameter and curvature of the lens because the lasing spot of a short-wavelength semiconductor laser is generally small and the lasing wavelength is so short at approximately 400 nm, and it is also extremely difficult to align the optical axes of the laser and the lens.
In addition, the diameter of the spot at the diffraction limit that can be focused by a lens is proportional to the wavelength, so that shortening the wavelength of the light source is an important technique in increasing the recording density. However, simply shortening the wavelength makes it impossible to focus the projected light to a tiny spot. In other words, it is necessary to develop some sort of lateral-mode control structure in order to obtain suitable beam characteristics. In general, crystalline growth and machining techniques have not yet been developed far enough for nitride semiconductors, however, and thus there is a problem in that it is difficult to implement a complicated lateral-mode control structure. In other words, there are still many problems to solve in the implementation of beam characteristics that can be used with an optical disc with a system that can achieve continuous room-temperature lasing using an InGaAlN laser.
If the wavelength of the light is short, moreover, precision control is required for the accuracy and adjustment of the optical system that is used. To reduce the wavelength and spot diameter in this case, aberration and other problems of the lens must also be controlled to small values. Thus problems increase as the wavelength shortens, making it difficult to implement and adjust such a high-precision optical system.
As discussed above, it is difficult to fabricate a lateral-mode control structure with a nitride semiconductor laser and it is difficult to implement beam characteristics that can be used for optical discs. When such a short-wavelength light-emitting element is used as a light source for an optical disc or the like, it is difficult to implement a high-precision optical system that is matched to the wavelength, and to adjust such an optical system with a high degree of precision.
The present invention was devised in the light of the above described problems and has as an objective thereof the provision of a light-emitting element that is provided with beam characteristics that make it suitable for use with an optical disc or the like.
The gist of this invention makes it possible to implement beam characteristics that are suitable for use in an optical disc system or the like, by the provision of a wavefront converter in a short-wavelength light-emitting element.
In other words, the light-emitting element of the present invention comprises a light-emitting portion made of a nitride semiconductor; and a first wavefront converter converting the radiating range of light that is emitted from the light-emitting portion into a radiating range that is smaller than the wavelength thereof, and outputting the same as output light.
In this case, if the first wavefront converter has a small aperture such as a pinhole that has a diameter that is smaller than the wavelength of the light that is emitted from the light-emitting portion, and the output light comprises an evanescent wave that is output to the exterior through this small aperture, it is possible to obtain an extremely small light spot.
The light-emitting element could be further provided with a transparent dielectric layer on the light-emitting surface of the small aperture.
If the light-emitting element is further provided with a transparent protective film that is coated onto an inner wall of the small aperture, it is possible to prevent the diameter of the small aperture from expanding unexpectedly if there is a sudden increase in current while the laser is being used in practice.
If the light-emitting element is further provided with a second wavefront converter for focussing light that is emitted from the light-emitting portion and supplying the same to the first wavefront converter, it is possible to further improve the evanescent output.
In this case, it is preferable that the second wavefront converter is any one of a concave reflective mirror, a Fresnel lens, a waveguide layer having a non-uniform spatial distribution of refractive indices, a planar reflective mirror, or a convex lens, for focusing light emitted from the light-emitting portion onto the small aperture.
In addition, the first wavefront converter could have a non-uniform spatial distribution of refractive indices, with the radiating range of light emitted from the light-emitting portion being output as output light after being converted into a radiating range that is smaller than the wavelength thereof, by a lens effect created by the spatial distribution of refractive indices.
In this case, the spatial distribution of refractive indices is created by varying effective refractive indices in a spatial manner, in accordance with a plasma effect achieved by varying the injection density of carriers in a spatial manner by adjusting values of resistivity within the first waveform converter.
This invention also relates to a surface-emitting type of light-emitting element comprising a multi-layered structure comprising a light-emitting layer; and a pair of electrodes for supplying a current to the light-emitting layer; wherein output light is output from a light-emitting surface of the multi-layered structure; and the pair of electrodes are provided in a recessed position from the light-emitting surface toward the light-emitting layer side. This makes it possible to bring the light-emitting surface extremely close to an object to be illuminated.
If the light-emitting surface is a surface of a wavefront converter having a small aperture of a diameter that is smaller than the wavelength of light emitted from the light-emitting layer; and the output light comprises an evanescent wave that is output to the exterior through this small aperture, it is possible to shine an evanescent wave reliably onto an object to be illuminated, by positioning the output surface of the evanescent wave sufficiently close to the target, such as an optical disc.
In this case, both of the pair of electrodes are provided on the same side, on either a top surface side or a rear surface side of the multi-layered structure.
The configuration could be such that one of the pair of electrodes and the light-emitting surface is provided on a main-surface side of the multi-layered structure and the other of the pair of electrodes is provided on a rear-surface side of the multi-layered structure, or the configuration could be such that the light-emitting surface is provided on the main-surface side of the multi-layered structure and both of the electrodes are provided on the rear-surface side of the multi-layered structure.
When the pair of electrodes are connected electrically to wires, if each of the electrodes is provided in such a manner as to not protrude on the side from which emitted light is extracted, it is possible to bring the light-emitting element sufficiently close to an object to be illuminated, to illuminate the object reliably with an evanescent wave.
It is also possible to improve the efficiency with which light is extracted, by further providing a transparent dielectric layer that is disposed on a light-emitting surface of the small aperture.
It is further possible to prevent the small aperture from expanding unexpectedly if there is a sudden increase in current while the laser is being used in practice, by providing a transparent protective film that is coated onto an inner wall of the small aperture.
If the light-emitting element is further provided with a second wavefront converter for focusing light that is emitted from the light-emitting layer and supplying the same to the small aperture, it is possible to improve the light output even further.
In this case, the second wavefront converter is preferably any one of a concave reflective mirror, a Fresnel lens, a waveguide layer having a non-uniform spatial distribution of refractive indices, a planar reflective mirror, or a convex lens, for focusing light emitted from the light-emitting portion onto the small aperture.
A method of fabricating a light-emitting element in accordance with the present invention, wherein the light-emitting element has a multi-layered structure comprising a light-emitting portion made of a nitride semiconductor, and a thin film in which is formed a small aperture having a diameter that is smaller than the wavelength of light emitted from the light-emitting portion; such that at least part of the light emitted from the light-emitting portion is produced as an evanescent wave through the small aperture, comprises the steps of: forming the multi-layered structure; forming the thin film on a surface of the multi-layered structure; and opening up the small aperture in the thin film in a self-aligning manner, by supplying a current to the light-emitting portion and illuminating light that is emitted from the light-emitting portion onto the thin film. This makes it possible to open up the small aperture, in an extremely easy and reliable manner, and also makes it unnecessary to use expensive equipment such as an FIB.
In this case, if the diameter of the small aperture is adjusted in the step of opening up the small aperture, by monitoring light that is projected through the small aperture with a detector, it is possible to control the diameter of the aperture easily and reliably.
Furthermore, if the diameter of the small aperture is adjusted in the step of opening up the small aperture, by monitoring light that is projected through the small aperture with a detector, it is unnecessary to place the detector too close to the light-emitting element.
This fabrication method could further comprise a step of coating an inner wall of the small aperture with a material that is transparent with respect to light that is emitted from the light-emitting portion, after the step of opening up the small aperture. This makes it possible to prevent the aperture from expanding unexpectedly if there is a sudden increase in current while the laser is being used in practice.
The effects achieved by the above described configurations are discussed below.
First of all, the present invention makes it possible to implement suitable beam characteristics, by providing a light-emitting element, which is made of a nitride semiconductor, and a wavefront converter.
In other words, it is possible to focus light from the light-emitting portion without using any form of optical system such as a lens, by producing an evanescent wave through the small aperture. As a result, the spot size of the thus obtained evanescent wave can be made no more than one-tenth the size of that in a conventional DVD system. This means that it is possible to implement an ultra-high-density optical disc system or a magneto-optical disc system that has a recording capacity that is at least one hundred times that of a conventional DVD system.
It is also unnecessary to adjust the lens to cope with changes in the wavelength, or adjust the optical axis within the pickup.
As described above, the present invention has many advantages from the industrial point of view in that it provides an ultra-high-density optical disc system that is inexpensive and highly reliable, by implementing a light-emitting element that has an extremely small spot size.